It is known from prior art that vanillin can be manufactured from solutions with a content of lignosulphonic acids and their salts, for example from spent sulphite liquor, by air oxidation of the substrate mixed with either sodium hydroxide or calcium hydroxide plus sodium carbonate. After the oxidation, the vanillin thus formed from lignin is present in the alkaline solution as water-soluble sodium vanillate. The vanillin can be isolated from the oxidised solution by acidifying for example with carbon dioxide and sulphuric acid, separating the lignin precipitated, and extracting the vanillin by the aid of a suitable solvent, e.g. benzene or toluene, (Hibbert, H., Tomlinson, G., Jr., U.S. Pat. No. 2,069,185, Jan. 26, 1937.
Methods are also known, according to which the vanillin is extracted from the oxidised solution as its sodium salt by using a higher alcohol (e.g. n-butanol or isopropanol) (Sandborn, L. R., Salvesen, J. R., Howard, G. C., U.S. Pat. No. 2,057,117, Oct. 13, 1936; Bryan, C. C., Can. Pat. No. 528,837, Aug. 7, 1956).
It is also known that carbonyl compounds can be isolated by the aid of cation exchangers, on which the resin first is allowed to react with certain nitrogen-containing compounds, for example with hydroxylamine and hydrazine, which alter the chemical character of the resin in a way such that it is capable of reacting quantitatively with the carbonyl compounds (Toppel, O., U.S. Pat. No. 2,897,238, July 28, 1959).
A method of isolation is also known by which the sodium salt of vanillin is transformed into vanillin by the aid of weakly protonised cation exchange resins, which at the same time are transformed into their sodium salt modification.
The inherent disadvantage of the methods described above is the need of neutralisation in order to isolate the vanillin, with concurrent precipitation of the lignin, extraction of vanillin from large volumes of dilute vanillin solutions containing large amounts of precipitated lignin, chemical transformation of the ion exchange resin into a modification that makes it possible to effect adsorption, or regeneration of the ion-exchange resin into its original ionic modification after each batch.